The present invention relates to systems and methods for magnetic resonance imaging (“MRI”) and, more particularly, to systems and methods for tracking a position of a medical device designed for intervention into a subject using an MRI system.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the nuclear spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. Usually the nuclear spins are comprised of hydrogen atoms, but other NMR active nuclei are occasionally used. A net magnetic moment Mz is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B1; also referred to as the radiofrequency (RF) field) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped” into the x-y plane to produce a net transverse magnetic moment Mt, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation field B1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance (“NMR”) phenomenon is exploited.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged experiences a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The emitted MR signals are detected using a receiver coil. The MRI signals are then digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
The lack of ionizing radiation and the ability to provide anatomical images of soft tissue with sufficient resolution makes MRI an appealing modality to couple with interventional medical procedures that can be performed less invasively or more efficiently or safely when guided by MR images. For example, the guidance of therapeutic devices, such as catheters, and/or the placement of interventional devices, such as guidewires and stents, using MRI guidance is a promising and evolving field with great clinical potential.
One particular challenge of this field, however, has been how to develop safe and reliable methods for tracking such devices as they are moved and manipulated within vessels or organs. The tips of guidewires can be easily visualized using conventional x-ray fluoroscopy by applying small, radio-opaque markers to the tips. Of course, x-ray fluoroscopy has the noted drawback of subjecting the patient and clinician to ionizing radiation. In MRI, the analog to the passive radio-opaque marker is a passive marker made of a material with a sufficiently large magnetic susceptibility, relative to the surrounding tissues, such as a stainless steel tip on a nitinol wire. In MR images depicting a guidewire containing such passive markers, a local hypointense region is present in the tissues adjacent to the markers, thereby resulting in a loss of clinically relevant information. Unfortunately, the high magnetic susceptibility of the material may induce artifacts in the MR images, among other drawbacks.
Accordingly, some other passive systems have been developed that have dedicated “indicator elements,” for example, including a paramagnetic material. In such devices a paramagnetic material may be integrated with a catheter or other interventional medical device to influence the magnetic resonance image of a patient to be examined by means of an MRI system. The influence the device has on MR images makes it possible to determine the position of the interventional instrument within the body of the patient without the instrument being directly visible. However, the influence of the paramagnetic component in the device on the MR image adversely affects the diagnostic quality of the magnetic resonance image. As a result of the influence of the indicator element, MR images will exhibit degraded or lost anatomical details in the regions adjacent to the indicator element.
Despite being easy to locate in MR images and relatively inexpensive and safe, the aforementioned interventional devices produce a loss of signal or otherwise induce artifacts in the vicinity of the interventional device that, in turn, obscures the desired region of interest, namely, the tissue adjacent to the tip of the device. Thus, while the location of the tips of the aforementioned devices can be easily identified, the nature of the tissue that the devices are being moved through is obscured by the same effect that allows the visualization of the devices.
Accordingly, some active systems have been developed. For example, some catheters and other interventional devices have been coupled with local coils or other mechanisms that provide active feedback regarding the position of the interventional device. These systems have the advantage of providing real-time, controllable feedback acquired from the perspective of the interventional device that can be coupled with the general anatomical data acquired by the general imaging of the subject using the MR system and non-interventional coils.
Such internal, coil-based real-time MR-guided interventional devices struggle to achieve a sufficiently high intrinsic signal-to-noise ratio (SNR) necessary to overcome the challenges posed by the real-time data acquisition needed to guide the intervention. A major problem of internal MR receiver coils is the low radial visualization obtained due to the extremely small coil diameter. For example, in interventional applications, these local, internal coils struggle to achieve a sufficient field of view (FOV) to visualize clearly the position of the interventional device. While providing beneficial feedback, these local coils are prone to motion artifacts because when the imaged anatomy is moving, these coils will move with the anatomy, thereby resulting in blurred images.
Also, these active, coil-based systems that are integrated or coupled with the interventional device present a number of additional operational constraints on the clinician. For example, the internal coils are coupled to the MRI system or other external systems through a cable that extends from the tip of the interventional device, out of the patient, and to the external connection point. Micro-coaxial cables are typically used for such connections to transmit the signal from the internal coil to the external systems. Unfortunately, such micro-coaxial cables are very lossy, and inject a significant amount of thermal noise to the signal detected by the internal coil. This often means that smaller signals received by the internal coil, for example, such as received from regions at a distance from the internal coil, are indistinguishable from the noise injected along the connecting cable, resulting in further reduced radial coverage.
It would therefore be desirable to have a system and method for tracking interventional devices using MRI that does not obscure anatomical images acquired by the MRI system, has a sufficiently accuracy to allow precise tracking of the interventional device, and is acceptable to clinical settings.